The present invention is an improvement over the invention in U.S. Pat. Nos. 4,294,078 and 4,403,645 of the present inventor.
There are a variety of applications which require the fast melt down of phase change material (PCM) from its solid, e.g. frozen or ice state, into its liquid, e.g. melted or water state, in order to provide an intense surge of cooling over a relatively brief time span. Examples of such applications include a short church service, brief use of an auditorium or theater, providing cool air to an airplane while it's unloading and loading passengers, levelling short term combinations of loads, and cooling computers or spaces during a brief power outage where the stand-by generator only needs to run a pump and blower but not the refrigeration equipment. An example of such a fast melt down application would be a large airport in which the central air conditioning system electrical load on certain summer nights coincides with and thus adds to the airport illumination electrical load for two hours before the air conditioning is shut off, thereby causing an expensive 2000 KW peak electrical demand, which would cost more than $50,000 per month in demand charges, that could be avoided by the use of fast melting stored ice.
Another example of such a fast melt down application would be an airlines terminal at a hub airport for many outlying cities, where more than a dozen passenger airplanes must simultaneously be provided with cool air for one hour while passengers are shifting to other flights to their respective outlying destinations. Then the gates will be empty for some time until another group of planes arrive together at the hub airport.
As a further example of an application calling for an intense burst of cooling for a brief period, it is noted that a computer installation requires the back-up of uninterruptible power systems (UPS) during a power failure so that their valuable memories will not be lost. To size UPS to provide cooling is very expensive. Computers heat up and become damaged very quickly if run without cooling, if even only for the time to store away memory content into magnetic discs.
In order to meet the requirements of these brief but intense cooling situations it would be possible to employ a relatively large number of the thermal storage tanks disclosed in U.S. Pat. Nos. 4,294,078 and 4,403,645 and then to operate all of their heat exchangers simultaneously in parallel with each other for obtaining their combined cooling effects. However, such attempted use of numerous tanks involves an expensive, large scale installation.
If an attempt is made to crowd more heat exchange tubing into a tank for increasing the cooling effect, an unexpected problem is encountered. Assuming that the PCM being used is water, the water surrounding the tubes becomes frozen completely solid when the tank is fully charged with cooling, thus, it will lose its buoyancy when all the liquid has become frozen. However, when the ice is nearly all frozen, say 90% or so, a maximum buoyancy force is equal to the weight of the displaced liquid less the weight of the displacing object, the buoyancy force at 90% frozen in water/ice is equal to 8.33 lbs/gallon times the approximately 9% expansion when ice is frozen. The heat exchanger tubes themselves, being made of a plastic slightly lighter than water and filled with an anti-freeze liquid solution slightly heavier than water, can be neglected. Thus, a tank holding 2,000 gallons of water will have a buoyancy force of 8.33.times.2,000.times.0.09.times.0.90=1,350 lbs on the tubes having a built-up ice coating.
In a phase change liquid salt solution having a density as high as 1.8 times the density of water, the buoyancy of the heat exchanger volume itself would also have to be taken into account, and thus the buoyancy forces can be much higher. If the heat exchanger, weighing the same as water, occupied 25% of the volume of the tank and the heavy phase change material did not change volume as it froze, the buoyancy force on the heat exchanger would be 0.25.times.2,000.times.(1.8-1.0).times.8.33=3,330 lbs.
When water freezes to solid ice or another phase change material freezes solid, crystals are formed which create strong local forces. Thus, I have found that a heat exchanger imbedded in such a crystallized mass must be somewhat flexible and resiliently deformable in its structure and, in its components to prevent breakage but also must have the inherent strength to resist the buoyance forces. Also, the components must be non-corrodible and inexpensive, since the heat exchanger would include many more components than in the above patents. Also, the component tubes and their support members are less likely to be deformed if the ice does not stick to their surfaces too strongly, as is noticed in an ice cube tray of a freezer where ice cubes can be dislodged by bending the tray. The material of which the tubes and support members are made must also not become brittle at low temperatures.